New “solar thermal fuel” has energy density of lead batteries

Store the UV energy in sunlight, release it as heat when needed.

Right now, photovoltaic devices are the cheapest, most efficient way to harvest the energy in sunlight. The problem is that this energy ends up in the form of electricity, which we have difficulty storing in a cost-effective manner. An alternative approach, solar thermal energy, converts solar energy to heat and can use that heat to continue generating power for several hours after the Sun goes down. But that's not enough to make solar an around-the-clock energy source.

Researchers are apparently working on a third option, one that could potentially store energy indefinitely. It goes by the name of "solar thermal fuel," but it's not a fuel in the traditional sense. Rather than breaking apart the fuel molecule through combustion, solar thermal fuels release heat by rearranging bonds within a molecule, leaving all the atoms in place. As a result, they can be recycled repeatedly—in the example that introduced me to solar thermal fuels, a research team ran theirs through more than 2,000 cycles with no loss in performance.

How do you get energy into and out of a molecule without breaking any bonds? In this case, the authors worked with derivatives of a chemical called azobenzene, shown below. The double bond between the two nitrogens forces the remaining bonds into one of two forms: either both of the rings can be on opposite sides of the molecule (top, called the "trans" form) or they can be on the same side (bottom, called "cis").

These two forms, called isomers, are like different energy states of the same molecule. The trans form can be considered the ground state; exciting it with UV light can flip the bonds, converting it to the cis form. The molecule stays locked in the cis form until some additional energy destabilizes it. It will then release energy in the form of heat as it switches back to the trans version. So individual azobenzene molecules can act a bit like a tiny energy storage system.

On its own, though, azobenzene isn't especially efficient at this. It tends to form a disorganized jumble of molecules, making it hard for individual ones to flex bonds. But a team of researchers at MIT did computer modeling that suggested the performance could be improved by up to 30 percent if they could get the azobenzene molecules lined up and stacked in a specific orientation.

To do that, the researchers chemically linked azobenzene to carbon nanotubes. The process wasn't especially efficient, but they could repeat it several times until the nanotubes had a healthy fuzz of azobenzene on their surfaces. The azobenzene could then be packed quite densely into a solid or kept in solution with a simple solvent like acetone.

The azobenzene still worked. Exposure to UV light would "charge" the system, and mild heating (75°C) in the dark was enough to get the chemical to release that energy in the form of heat. And using carbon nanotubes to pack the molecules in succeeded in boosting the amount of energy that could be stored in the system, even though the nanotubes were non-functional and accounted for nearly half of the bulk weight of the storage medium.

The surprise, however, was how much the nanotubes boosted the performance. Rather than the expected 30 percent increase in energy storage, they saw a boost of 200 percent.

With that large of an increase, the numbers on the system became quite good. The authors found that, given UV light, the energy storage efficiency of the material was about 14 percent. The material could be cycled through charge and release of energy at least 2,000 times without any drop in performance, and it could stably store energy for extended periods of time. The energy density was quite good as well, at 44 Watt-hours/kg, making it similar to a lead-acid battery.

The process the researchers used to link the azobenzene to the nanotubes doesn't result in an even coating, and it was possible to detect areas in the bulk material that stored energy more or less efficiently. So figuring out an optimal density (and figuring the chemistry needed to produce it) is probably a priority.

What isn't clear from this work is the sorts of temperatures that this material can produce—specifically, is the heat concentrated enough to boil water? Can it release enough heat to boil water without starting to decompose? Since heat itself can trigger the reverse reaction, it's also worth considering whether there's a danger of random temperature spikes setting off a chain reaction in the storage medium.

Still, the performance of this material seems to make those issues worth looking into. Having an additional option for storing solar power could provide a great deal of flexibility—collectively, the different forms of solar could keep the Sun's energy flowing around the clock.

Promoted Comments

At sufficiently high functionalization densities (Azo-SWCNT 3), an absorbance band at ~350 nm, attributable to the π → π* transition of the appended azobenzenes, appears in the optical absorbance spectra of dilute acetonitrile suspensions.

That's still in the UV, but close enough to visible that there's still a fair amount of it in sunlight at ground level.

For the non-chemists/physicists: in order for the molecule to switch from one form to the other, it has to be able to freely rotate about the central N-N bond. To do this, you have to temporarily break the second N-N bond by splitting up the electron pair that forms it by exciting one of those electrons to a higher energy level. (The same idea as the more familiar electron excitation in atoms that leads to absorption bands in the solar spectrum, but involving molecular rather than atomic orbitals).

Since the excited state is more unstable, it will have a certain lifetime during which the rotation can occur. When the electron relaxes back to its original energy, the second bond reforms and further rotation is prevented. The energy storage results from repulsion between the two rings either side of the N-N bond when they are on the same side of the molecule. The rest, as they say, is (quantum) thermodynamics.

It appears that the UV wavelengths cause the azobenzene to go from the ground to "excited state," and a mild amount of applied heat will cause the "battery" to release the stored energy in the form of heat output - hopefully useful heat that can boil water to run a steam turbine for electricity. I think the major hurdle is that the battery needs to be thermally isolated in order to prevent any temperature spikes from causing an accidental release of thermal energy.

This appears to me to limit this technology to a fixed installation, rather than a mobile application. But if it can boil water, this has the potential for a new alternative power plant - if it scales up well thermodynamically and economically.

Isn't it a bit misleading to compare the energy storage density of this medium with that of lead-acid batteries? The energy you would get, in the form of heat, would have to go through another lossy process to be converted to electricity.

It's apples-and-oranges to compare the energy storage density of media yielding energy of such different levels of entropy.

An alternative approach, solar thermal energy, converts solar energy to heat and can use that heat to continue generating power for several hours after the Sun goes down. But that's not enough to make solar an around-the-clock energy source.

Huh? According to wikipedia molten salt thermal energy storage can hold it's heat, up to 500C, for more than a week. And it can generate energy until it reches 130C before it becomes solid and needs to be heated up again.

Personally I see this, or something like it, as the best option for power plants in warmer parts of the world. With a fossil fuel (or biofuel?) backup in place when a week isn't long enough. It's simple and cheap and we know how to work with it.

Molten salt is already used to store heat in the manufacturing industry, and nuclear plants generate power by converting heat into steam. Creating molten salt is simple, just point a big mirror at it.

I agree we need to be doing research into better technology, but we don't need a high tech futuriatic inventions to generate most of our energy from clean sources. We need to start building power plants based on currently available and proven options.

Yeah my immediate thought was "how hot can it get" when decomposing and since heat causes it to decompose, releasing more heat, is this going to be a runaway chain reaction risk.

The downside I see here is, the heat density might be pretty good, but you have to convert it back to usable electricity. That takes just a wee bit more. thermal photovoltaic and thermal electric systems just aren't very efficient, which reduces net system efficiency significantly, and even something like a stirling engine isn't all that efficient (50% at best) and could add a lot of cost to a system.

I could see it as a viable method of both hot water heating as well as winter whole house heating though. And THAT could be pretty significant for a lot of the world. If efficiencies can be boosted enough, then low efficiency recovery systems might not matter nearly as much, especially if the life time of the storage medium is very long compared to batteries.

Something you can install and "forget" about for decades and decades is a lot better than any battery that might have a life span of 5-10 years. Tops.

At sufficiently high functionalization densities (Azo-SWCNT 3), an absorbance band at ~350 nm, attributable to the π → π* transition of the appended azobenzenes, appears in the optical absorbance spectra of dilute acetonitrile suspensions.

That's still in the UV, but close enough to visible that there's still a fair amount of it in sunlight at ground level.

For the non-chemists/physicists: in order for the molecule to switch from one form to the other, it has to be able to freely rotate about the central N-N bond. To do this, you have to temporarily break the second N-N bond by splitting up the electron pair that forms it by exciting one of those electrons to a higher energy level. (The same idea as the more familiar electron excitation in atoms that leads to absorption bands in the solar spectrum, but involving molecular rather than atomic orbitals).

Since the excited state is more unstable, it will have a certain lifetime during which the rotation can occur. When the electron relaxes back to its original energy, the second bond reforms and further rotation is prevented. The energy storage results from repulsion between the two rings either side of the N-N bond when they are on the same side of the molecule. The rest, as they say, is (quantum) thermodynamics.

An alternative approach, solar thermal energy, converts solar energy to heat and can use that heat to continue generating power for several hours after the Sun goes down. But that's not enough to make solar an around-the-clock energy source.

Huh? According to wikipedia molten salt thermal energy storage can hold it's heat, up to 500C, for more than a week. And it can generate energy until it reches 130C before it becomes solid and needs to be heated up again.

Personally I see this, or something like it, as the best option for power plants in warmer parts of the world. With a fossil fuel (or biofuel?) backup in place when a week isn't long enough. It's simple and cheap and we know how to work with it.

Molten salt is already used to store heat in the manufacturing industry, and nuclear plants generate power by converting heat into steam. Creating molten salt is simple, just point a big mirror at it.

I agree we need to be doing research into better technology, but we don't need a high tech futuriatic inventions to generate most of our energy from clean sources. We need to start building power plants based on currently available and proven options.

I believe the promise of the article though is onsite storage in a residential or small commercial/industrial setting.

You aren't going to be using big solar concentrators on top of your home to use molten salt as a storage medium.

Distributed solar is one of the more promising ways to deploy renewables, especially if a resonable, reliable, inexpensive and efficient means of storing that energy for night time use and cloudy day use can be developed. Then grid flexibility is much less of a thing.

An alternative approach, solar thermal energy, converts solar energy to heat and can use that heat to continue generating power for several hours after the Sun goes down. But that's not enough to make solar an around-the-clock energy source.

Huh? According to wikipedia molten salt thermal energy storage can hold it's heat, up to 500C, for more than a week. And it can generate energy until it reches 130C before it becomes solid and needs to be heated up again.

Personally I see this, or something like it, as the best option for power plants in warmer parts of the world. With a fossil fuel (or biofuel?) backup in place when a week isn't long enough. It's simple and cheap and we know how to work with it.

Molten salt is already used to store heat in the manufacturing industry, and nuclear plants generate power by converting heat into steam. Creating molten salt is simple, just point a big mirror at it.

I agree we need to be doing research into better technology, but we don't need a high tech futuriatic inventions to generate most of our energy from clean sources. We need to start building power plants based on currently available and proven options.

I agree that we have many options to build power plants from existing technology, but how do you think that existing technology came to exist? All new technology at one point was not known and had to be discovered.

Is it strictly UV light that triggers the transformation? What percentage of "normal" sunlight is UV? Either way, this sounds very, very cool.

Looks to me like a few percent of light is at the right wavelengths so .14*.05 yields a real solar conversion rate of less than 1%. Now if you couple a high efficiency solar power generator to a UV laser you might do a little better but probably not much at getting the energy in.

An alternative approach, solar thermal energy, converts solar energy to heat and can use that heat to continue generating power for several hours after the Sun goes down. But that's not enough to make solar an around-the-clock energy source.

Huh? According to wikipedia molten salt thermal energy storage can hold it's heat, up to 500C, for more than a week. And it can generate energy until it reches 130C before it becomes solid and needs to be heated up again.

Personally I see this, or something like it, as the best option for power plants in warmer parts of the world. With a fossil fuel (or biofuel?) backup in place when a week isn't long enough. It's simple and cheap and we know how to work with it.

Molten salt is already used to store heat in the manufacturing industry, and nuclear plants generate power by converting heat into steam. Creating molten salt is simple, just point a big mirror at it.

I agree we need to be doing research into better technology, but we don't need a high tech futuriatic inventions to generate most of our energy from clean sources. We need to start building power plants based on currently available and proven options.

I believe the promise of the article though is onsite storage in a residential or small commercial/industrial setting.

You aren't going to be using big solar concentrators on top of your home to use molten salt as a storage medium.

Distributed solar is one of the more promising ways to deploy renewables, especially if a resonable, reliable, inexpensive and efficient means of storing that energy for night time use and cloudy day use can be developed. Then grid flexibility is much less of a thing.

The thing I worry about is safety. Most benzene like compounds are not good to be around. For home systems what happens when the tank leaks?millions of gallons of this leaking into groundwater might not be a good thing.

I believe the promise of the article though is onsite storage in a residential or small commercial/industrial setting.

You aren't going to be using big solar concentrators on top of your home to use molten salt as a storage medium.

Distributed solar is one of the more promising ways to deploy renewables, especially if a resonable, reliable, inexpensive and efficient means of storing that energy for night time use and cloudy day use can be developed. Then grid flexibility is much less of a thing.

As someone who lived off photovoltaic solar for 5 years and has a solar hot water system on the roof now, I think distributed solar is a terrible idea.

It's a nice pipe dream but it just isn't cost effective.

Efficient energy generation needs economy of scale to be efficient. Especially if you want to dramatically increase home energy needs by charging your electric cars at home and switching everyone to electric heating for their house to reduce emissions (which is the goal right?) - and we need heating in winter when solar is already pretty inefficient.

I wrote a whole proposal for my doctorate using azobenzene derivatives as a way to control polymerization. They are an interesting class of molecule with a rather convenient light switch in the N=N double bond.

Not that this isn't cool technology or something worth pursuing, but...

Why did I know before clicking the link that Carbon Nanotubes would be involved? They need to engineer this like a Kalashnikov - wide tolerances such that it won't end up turning into some Ozone-Layer-Destroying chemical byproduct or Zombie Apocolypse.. http://en.wikipedia.org/wiki/Carbon_nanotubes#Toxicity

And the safety of the chemical itself (thermal runaway notwithstanding)?

Well, you always need to be a little concerned about any molecule with N-N bonds because of the potential for releasing molecular nitrogen (usually explosively). But azobenzene is pretty stable. The pi electrons in the benzene rings stabilize it (and also shift the pi-pi* transition into the UV range).

According to a talk I saw by Murray Gell-Mann, if you are going to fool around with carbon nanotubes for energy storage, the route to the future is in direct storage of electrons (electricity) in the nanotubes. He claimed a small set of nanotube batteries could power a household all night, while recharging through PV during the day. Unfortunately, he was rather pessimistic as to the prospects, although he admitted he had been surprised at advancing technology before.

As to the centralized/distributed energy generation debate, let us not forget that 50% or more of energy is lost during production and, importantly, distribution. The closer the source to the use, the better.

An alternative approach, solar thermal energy, converts solar energy to heat and can use that heat to continue generating power for several hours after the Sun goes down. But that's not enough to make solar an around-the-clock energy source.

Huh? According to wikipedia molten salt thermal energy storage can hold it's heat, up to 500C, for more than a week. And it can generate energy until it reches 130C before it becomes solid and needs to be heated up again.

Personally I see this, or something like it, as the best option for power plants in warmer parts of the world. With a fossil fuel (or biofuel?) backup in place when a week isn't long enough. It's simple and cheap and we know how to work with it.

Molten salt is already used to store heat in the manufacturing industry, and nuclear plants generate power by converting heat into steam. Creating molten salt is simple, just point a big mirror at it.

I agree we need to be doing research into better technology, but we don't need a high tech futuriatic inventions to generate most of our energy from clean sources. We need to start building power plants based on currently available and proven options.

I believe the promise of the article though is onsite storage in a residential or small commercial/industrial setting.

You aren't going to be using big solar concentrators on top of your home to use molten salt as a storage medium.

Distributed solar is one of the more promising ways to deploy renewables, especially if a resonable, reliable, inexpensive and efficient means of storing that energy for night time use and cloudy day use can be developed. Then grid flexibility is much less of a thing.

The thing I worry about is safety. Most benzene like compounds are not good to be around. For home systems what happens when the tank leaks?millions of gallons of this leaking into groundwater might not be a good thing.

Oh, THIS particular chemical might be a horrible idea (I have no idea). However, the concept behind it I think is the point. Something that you can turn the sunlight that hits a collector that can be placed on your roof, that can use some portion of that energy for your daytime energy needs and then store excess energy for night time/cloudy day use is needed. This would turn photovoltaic and solar thermal systems from a nice little supplement to things like coal and natural gas plants to something that can truely replace them.

As it stands, battery technology isn't nearly close enough on cost and storage density. Something like low cost and long life sodium air batteries might well be the ticket.

However, who knows exactly what technology we might develop that would meet our needs.

Maybe its a two prong answer. Something that can store sunlight in chemical/electrical form, like an advanced battery for electrical needs and something that can store it in chemical/thermal form for water and residential heating.

I wrote a whole proposal for my doctorate using azobenzene derivatives as a way to control polymerization. They are an interesting class of molecule with a rather convenient light switch in the N=N double bond.

I'll bet there are some really interesting tricks you can play with it, considering the difference in dipole moment (and infrared spectra) of the two isomers.

I believe the promise of the article though is onsite storage in a residential or small commercial/industrial setting.

You aren't going to be using big solar concentrators on top of your home to use molten salt as a storage medium.

Distributed solar is one of the more promising ways to deploy renewables, especially if a resonable, reliable, inexpensive and efficient means of storing that energy for night time use and cloudy day use can be developed. Then grid flexibility is much less of a thing.

As someone who lived off photovoltaic solar for 5 years and has a solar hot water system on the roof now, I think distributed solar is a terrible idea.

It's a nice pipe dream but it just isn't cost effective.

Efficient energy generation needs economy of scale to be efficient. Especially if you want to dramatically increase home energy needs by charging your electric cars at home and switching everyone to electric heating for their house to reduce emissions (which is the goal right?) - and we need heating in winter when solar is already pretty inefficient.

You know what an economy of scale is? Putting it on the roof of every house. PV is already starting to edge towards cost effective compared to traditional generation methods and a big part of the cost is initial install. If it gets cheap enough and a good storage method can be developed, it isn't a stretch to just mandate its instalation on new construction. As it is, the cost of a complete PV system is rather cheap compared to the cost of a new house. Even a cheap house is going to be over $100,000 and adding something like a 4kw PV system DURING construction, might only add $10,000 to the price, on the high end. If prices come down further and you are talking just 3-4% of a cheap home, that is getting in to the insignificant territory, especially if you consider that it'll effectively cut the cord on the electric utility for all future homeowners (or at least greatly reduce dependance).

It would need to be coupled with good storage though for it to be worth while in its effect on the overall electric grid beyond supplementing a few percent of the generation capacity though.

For an EV, a roof can already generate enough power for a residence and an EV or two.

Lets assume the house uses 1000kwh a month and the EV gets 3 miles to the kwh, with an average round trip commute of 30 miles, 30 days a month (lets assume weekend driving is the equivelent of the weekday commute). That EV would roughly double the residence's power requirements to about 1,900kwh a month. Most places in the US get between 4-6.5 insolation hours per day (that is the average amount of full sunshine received in a day, averaged over the year, taking in to account average cloud cover, day/night cycle, etc).

Average US house size is about 2160sq-ft. Average homes are single story in the US still, but lets halve it anyone to take in to account all of the townhouses and two story homes in there. With a 30 degree roof pitch and half facing the sun, that gives you 810 sq-ft of roof on the average house.

Just quick checking some panels on Amazon and it looks like around 15sq-ft for a 200w panel. That gets you 810sq-ft roof with about 10.8kw on the solar array at current technology.

So about 40-65kwh of production per day, times 30 is around 1200-1950kwh in a month. That is assuming a typical not too efficient US home (not a newer and/or more energy efficient home), as well as something like the Tesla model S, which is not an overly efficient EV or the fact that EVs are likely to improve over time.

Prices go down, efficiency goes up and it becomes extremely viable. Throw in storage technologies that work and are viable in terms of space, cost and longevity and BOOM. Solar takes off like crazy and solves all of the worlds energy problems.

Of course that is just a pipe dream of mine and doesn't mean it'll ever happen. But it might.

And the safety of the chemical itself (thermal runaway notwithstanding)?

Well, you always need to be a little concerned about any molecule with N-N bonds because of the potential for releasing molecular nitrogen (usually explosively). But azobenzene is pretty stable. The pi electrons in the benzene rings stabilize it (and also shift the pi-pi* transition into the UV range).

...and by using larger aromatic groups than benzene, you could probably tune it to use visible light.

It's not so much that we need to start building these things everywhere, but more that we should start looking at different variations on this theme.

Is it strictly UV light that triggers the transformation? What percentage of "normal" sunlight is UV? Either way, this sounds very, very cool.

Looks to me like a few percent of light is at the right wavelengths so .14*.05 yields a real solar conversion rate of less than 1%. Now if you couple a high efficiency solar power generator to a UV laser you might do a little better but probably not much at getting the energy in.

This reminds me of a cringe-worthy scene I saw in a SciFi series (Space Above and Beyond for the IMDB geeks). Apparently the standard procedure in setting up a radio was to shine flashlights at its solar panels. No wonder the humans were getting their asses kicked.

Question for the chemistry experts: is it generally true that cis molecules are at a higher energy state then their trans counterparts, or is that just accidentally true in the case of this particular molecule?

I believe the promise of the article though is onsite storage in a residential or small commercial/industrial setting.

You aren't going to be using big solar concentrators on top of your home to use molten salt as a storage medium.

Distributed solar is one of the more promising ways to deploy renewables, especially if a resonable, reliable, inexpensive and efficient means of storing that energy for night time use and cloudy day use can be developed. Then grid flexibility is much less of a thing.

As someone who lived off photovoltaic solar for 5 years and has a solar hot water system on the roof now, I think distributed solar is a terrible idea.

It's a nice pipe dream but it just isn't cost effective.

Efficient energy generation needs economy of scale to be efficient. Especially if you want to dramatically increase home energy needs by charging your electric cars at home and switching everyone to electric heating for their house to reduce emissions (which is the goal right?) - and we need heating in winter when solar is already pretty inefficient.

Just wondering what the aspect of using hydrolisis would be in that you would store your own in tanks,and retrieve it when you needed it. (Thus taking energy from one form,and putting it into another).

Like the system of solar to electric. But of course the situation is storage. If the hydrogen was storable,and then usable . Know there is problems of storing hydrogen. But there has been several solutions. Thanks for reading.

Question for the chemistry experts: is it generally true that cis molecules are at a higher energy state then their trans counterparts, or is that just accidentally true in the case of this particular molecule?

Generally that's the case, although I don't think exceptions are strictly forbidden.

Does this method hold more promise than existing phase change materials? Also, how can anything be simpler and more reusable than heating water (or metal salts or anything with enough thermal capacity) for storing solar energy?